1. Field of the Invention
The invention relates to a horn antenna for a radar device including a metal body having a hollow horn section that expands to an open end in the metal body, a dielectric filling body arranged in the hollow horn section, and a dielectric cover attached to the filling body and sealingly closing the open end of the metal body, where the dielectric filling body includes a first conical portion that is situated towards the open end of the metal body and is fittingly received in the hollow horn section, and where a second conical portion that is situated away from the open end and is dimensioned to leave a gap between itself and the wall of the hollow horn section.
2. Description of the Related Art
U.S. Pat. No. 6,661,389 discloses a horn antenna. By means of a horn antenna, which also is known as a cone antenna, microwave pulses are radiated, which have been generated by high frequency (HF) energy coupled in. In a combined transmitting and receiving system of a level measuring device equipped with such an antenna, the pulses reflected by a filling product are detected, and the distance from the filling product is assessed by measuring the transit time of these pulses. Radar-based level measuring devices are, for example, used for a continuous level measurement of fluids, as well as of bulk goods or a combination of such products.
For antennas that are not exposed to a heavy chemical load, metallic horns preferably of stainless steel are used. For highly aggressive process environments or in applications in which the filling product to be measured is, for purity or hygienic reasons, not allowed to come into contact with metal, it is known to provide a suitable encapsulated antenna.
A horn antenna comprising a metal body, preferably of aluminum, is depicted in FIG. 8 of the above-mentioned U.S. Pat. No. 6,661,389 in which a cone-like hollow horn section is formed. The metal body is screwed in the opening of a mounting flange of a vessel, where the open end or aperture of the horn section is arranged flush with the opening. The inner space of the horn section is filled with a dielectric filling body that is assembled from three different parts, where one of the parts is formed as a disk that covers and seals the opening against the environment inside the vessel and, in its middle portion, forms a convex microwave lens. The other parts are formed as a truncated cone and a pointed cone, where the pointed cone features such an outer dimension that between its outer wall and the inner surface of the horn section a minimal gap remains to compensate for expansion variations conditioned by temperature influences. The truncated cone part is bonded by adhesion with the metal body, in particular by a silicone adhesive for attachment of the dielectric filling body.
US 2009/0212996 A1 discloses another conventional horn antenna, but differing from the horn antenna described in U.S. Pat. No. 6,661,389 in that the dielectric filling body is integrally formed. The dielectric filling body has a cylindrical section that is inserted in a tubular waveguide section of the metal body and fixed at that point by sealing and locking means, thus preventing the filling body from falling out of the horn section of the horn antenna. The dielectric material of the filling body has a higher coefficient of thermal expansion than the metal body. Accordingly, a circumferential gap is provided between the outer surface of the dielectric filling body and the inner surface of the horn section. An alternative or supplemental sealing and locking element between the filling body and the metal body may be provided in the region of the aperture of the horn section. The locking element such as a round or flat wire, a Seeger ring, a retaining ring of spring steel is so formed that it does not produce disturbing microwave reflections.